OddMuse

OddMuse is by far one of the best wikis around. Of course this depends on your definition. Mine is about customisation and simplicity. I want to be able to completely control the look and feel, have up to 1000s of wikis per domain and be able to add any number of modules. This is not possible with many wikis which are designed for domain specific purposes (e.g. MediaWiki - excellent as the WikiPedia wiki, or Twiki an excellent Tech team tool).